Coated electrical insulating paper and method of making it

ABSTRACT

Electrical insulating paper bearing a surface coating of a material insoluble in liquid dielectrics and substantially free of alkali metals. The coating material is present to the extent of between 2 and 15 percent of the total weight of the coated paper. The coated paper is thinner than otherwise identical uncoated paper having the same dielectric strength, and the coated paper has a percentage increase in dielectric strength greater than the percentage increase in its weight due to the presence of the coating. The coating material is soluble in water within a first range of temperatures and insoluble in water within a second range of temperatures. Prior to application of the coating material to the paper, the material is washed in water within the second range of temperatures to remove alkali metals, and thereafter the material is dissolved in water within the first temperature range to produce an aqueous solution for application to the paper. The coating material may be methyl cellulose, starch, polyvinyl alcohol, or protein.

United States Patent Suter et al. 1 Sept. 2, 1975 [541 COATED ELECTRICAL INSULATING 2,993,949 7/1961 Moebius ct a1. 1 17/68 X 3,067 141 12/1962 81103168 Ct 211. 1 1 17/157 3582513 6/1971 Bouchard ct a1. 2611/91.} X [75] Inventors: Kurt Suter, Carlisle, Pa; John H.

Matthews Mass Primary Examiner-Michael R. Lusignan 7 Assignee; Ki b l -cl k Corporation, Attorney, Agent. or Firm-Alan H. Levine, Breiten- Neenah, Wis feld & Levine [22] Filed: Apr. 5, 1971 [57] ABSTRACT 1 1 p 131,060 Electrical insulating paper bearing a surface coating of a material insoluble in liquid dielectrics and substan- 5 s Cl. H 42 /47 174/110 P; 174/12 B; tially free Of 811(811 metals. The coating material 18 252/63; 260/913 VA; 428/511; 533; present to the extent of between 2 and 15 percent of 428/536 the total weight of the coated paper. The coated paper [51] hm 12 B32B B32B 23/08; is thinner than otherwise identical uncoated paper 3323 23/O4;H01B 7/00 having the same dielectric strength, and the coated [58] Field of Search 117/60, 68, 155 UA, 156 paper has a percentage increase in dielectric strength 1 17/157, 16] U; 174/1 P, 121 B; greater than the percentage increase in its weight due 260 23 R 913 PV 913 VA; 252 3 t0 the presence Of the coating. The coating material 18 428/476 51 1, 533 536 soluble in water within a first range of temperatures and insoluble in water within a second range of tem- [56] References Cited peratures. Prior to application of the coating material UNITED STATES PATENTS to the paper, the material is washed in water within the second range of temperatures to remove alkali 1.733524 10/1929 Bradner t. 117/156 metals. and thfireafter the material is dissolved in Egg ss l water within the first temperature range to produce an 2 3 4/1950 117/157 aqueous solution for application to the paper. The 2526330 10/1950 Clark 117/113 coating material may be methyl cellulose, starch, poly- 2,642,420 6/1953 Kenyon et al.. 260/913 vinyl alcohol, of P 2,7l2,539 7 1955 Nu ent et al.. 117/156 X 2,926,088 2/1960 Spifclman 99 71 Clams 4 Draw'ng F'gures (72547: a) I- [600 v1 y is I a Mae 1 I g 5 E l g I {aura-aria) k1 g [000 l/ I I i a Q I '1 3 309 g t E t t e "1 a 16 g w 6 g 600 11 a h 3 Q s u "a 5 4 Q a 2 a 9 Q Q g g E s 3 2a \a as 0-7 as 119 10 COATED ELECTRICAL INSULATING PAPER AND METHOD OF MAKING IT This invention relates generally to paper, and has particular reference to paper used as electrical insulation.

Paper intended for use in electrical insulating applications where high dielectric strength is required, as in capacitors for example, is made on Fourdrinier papermachines, using specially purified, very highly beaten kraft woodpulp. Such paper is essentially film-like in character, i.e., it is substantially free of holes and of internal voids, and has extremely low permeability to fluids. However, unlike films made from homogeneous systems, e.g., cellophane and cast or extruded plastic films, the surface of capacitor paper is rough or toothy on a microscopic scale. This fine scale roughness is an important advantage in the use of paper as electrical insulation, since it provides passages for the ingress of liquid impregnants between the layers of insulation. However, whenever two microscopic valleys on opposite sides of the paper happen to occur at the same position, a thin spot results which represents a point of dielectric weakness and potential dielectric failure.

It is a general object of the present invention to increase the dielectric strength of electrical insulating paper without however increasing its thickness. Maintaining the paper thin is important should it be used as a dielectric spacer in a capacitor, since as is known the smaller the dielectric spacing between the electrodes of a capacitor, the greater will be its capacitance. The advantage of paper insulation having maximum dielectric strength with minimum thickness is not limited to capacitors, but is generally useful in making electrical devices. For example, such paper wrapped around the wires forrnng the windings of transformers permit the size of a transformer to be reduced without reducing its power-handling capcity.

The invention is predicated upon the discovery that the application to the paper surface of dilute solutions of certain water-soluble, film-forming polymers produces dramatic increases in the dielectric strength of the paper. For example, an amount of coating which increases the weight per unit area of the paper by less than 4 percent, can increase dielectric strength by as much as percent. Put another way, the improved paper of a given total basis weight, including both fiber and surface coating according to this invention, is superior in dielectric strength to conventional paper of the same weight, composed entirely of fiber.

This entirely unexpected result of such low levels of coating material presumably indicates that the polymer solution tends to concentrate in the valleys in the paper surface but, if this is the case, the levelling effect is not such that the surface tooth is reduced suf ficiently to signficantly interfere with the impregnation of paper insulation with liquid dielectrics.

Thus, the invention is to be distinguished from socailed lacquered insulating or capacitor paper wherein a continuous coating of material is applied to the paper surface thereby increasing the effective thickness of the original paper.

There are, of course, a great many materials available which are both water soluble and film-forming. How ever, to be useful in the practice of this invention, the material must be insoluble in the liquid dielectrics commonly used to impregnate paper insulation, such as chlorinated biphenyls, mineral oil, etc.; it must not itself cause an intolerable increase in the dielectric loss factor of the paper; and it must be substantially free of alkali metals, because the presence of such metals increases the power factor of the paper. Since we have found all otherwise appropriate materials to contain excessive quantities of sodium, we have developed a commercially practical process for removing alkali metals from certain potentially useful, water-soluble, filmforming materials, and the requirements of the removal process define and limit the materials which are useful to us, apart from the other criteria described above.

Thus, to serve our purpose the film-forming material must be one which is insoluble in water within one temperature range but which dissolves within some different temperature range. For ease of processing, both of these temperature ranges preferably lie below l0OC but this is not essential. By insoluble we mean that the material must not become highly swollen (gelatinized) or tacky, but it must be permeable to water so that diffusion of ions from the solid into the water phase can take place. Materials having this critical characteristic permit the easy removal of alkali metals by simply washing them with water of low alkali metal content, maintaining the temperature within the range in which dissolution cannot occur. The efficiency of the washing process is improved by adding very small amounts of harmless cations, such as hydrogen ions or alkaline earth ions, to the wash water. These cations do not impair the electrical properties of the filmforming material as do alkali metal ions, and they serve to displace alkali metal ions from cationexchange sites in the material, thereby greatly reducing the amount of washing required to effect the desired reduction in alkali metal content. We find it advantageous to provide the cations in the early stages of the washing process by adding magnesium sulfate or sulfuric acid to the wash water in the amount of about 0.01 to 0.1 percent by weight of the water. The washing process is also facilitated by having the material to be washed in a finely divided state (high surface to volume ratio) so that the rate of diffusion of ions from the solid into the liquid phase will be rapid.

The washing can be carried out by any of a number of well established processes for washing finely divided solids, such as repeated batchwise dispersing, settling, and decantation; continuous washing and centrifugal decantation; and counter-current washing with rotary filters. Whatever the technique, extraction must be continued until the alkali metal content of the filmforming material, which will typically be about 1,000 parts per million before washing, has been reduced to no more than about 200 parts per million, and preferably, to no more than about 20 parts per million. After washing has been completed, at a temperature at which insolubility of the material is maintained, the material is then dispersed in the desired quantity of water which is substantially free of alkali metals and the temperature is then raised or lowered, as the case may be, to a point within the range where solubility occurs. After dissolution is complete, any required adjustment of the concentration and final temperature is made, and the solution is ready to be used in the manufacture of our improved paper.

Four materials which meet the criteria for our use are methyl cellulose, starch, polyvinyl alcohol, and protein.

Methyl esters of cellulose are manufactured in the US. by Dow Chemical Company under the trademark Methocel." These products are derived from either cotton linters or woodpulp and are available in wide ranges of viscosity and degree of methoxylation. We prefer grades having viscosities between 10 and 25 centipoise (2 percent aqueous solution, 20C) and a degree of substitution between 1.64 and 1.92. This type of methyl cellulose is insoluble in organic solvents and has the unusual property of being insoluble in hot water but readily soluble in cold water. Accordingly, the washing process with this material is carried out at a temperature of about 80C, and the slurry of washed methyl cellulose is then chilled to C in order to obtain a gel-free solution. After the methyl cellulose has dissolved, solubility is retained if the solution temperature is increased, up to about 35C, and the viscosity of the solution is reduced. The optimal temperature for application to the paper is about 30C.

The starches we find most useful are the oxidized and the chemically substituted types, such as hydroxyethylated starch, acetylated starch, etc., which are commonly used as surface sizes and coating binders in the manufacture of printing and writing papers. Enzyme converted pearl starches can also be used, but they are less convenient and the rheology of their solutions is less desirable than that of the chemically modified starches. These starches are usually derived from corn but are also made from potatoes, milo, tapioca, etc. We prefer the low and medium viscosity grades having viscosities below about 200 centipoise (10 percent aqueous solution, C). These starches are insoluble in cold or warm water but dissolve when cooked at about 90C for 20 to 30 minutes. Once dissolved, the starch remains in solution on cooling, and the solution temperature can be lowered for application to the paper. The optimal temperature for application can be anywhere between about 30C and 90C, depending upon the solution concentration and the viscosity of the starch.

Polyvinyl alcohol is available in types which are soluble in water over the whole temperature range between 0C and 100C, but the type which we employ is the socalled fully hydrolyzed which has about 98 percent or more of the original acetyl groups removed by hydrolysis. This type of polyvinyl alcohol is similar to starch in its solubility characteristics, i.e., it is insoluble in water at moderate temperatures but dissolves on cooking at 80C, depending upon the molecular weight and the number of residual acetyl groups. We prefer the products with viscositics between about and 125 centipoise (4 percent solution, 20C). At lower viscosity or degree of hydrolysis, some cold water sensitivity can occur, which may make the washing process more difficult.

Protein for use in our improved paper dielectric can be derived from any source provided that it is in a form which shows the solubility behaviour required for washing. The product which we prefer is purified soya protein, commonly known as alpha protein." The low viscosity version of this material is the most suitable. Unlike the other three materials described above, protein requires the addition of ammonium hydroxide to a pH .of about 8 9, as well as heating to about 50C or higher in order to obtain solubility.

All four of these water-soluble, film-forming materials are about equal in their ability to increase the electric strength of insulating papers. However, polyvinyl alcohol and protein increase the dissipation factor of the treated paper a small amount and, for this reason, these two materials are less suitable then methyl cellulose and starch in applications such as the making of capacitors, where this property of electrical insulation is critical.

Methyl cellulose, in addition to increasing the electric strength of paper, has a unique ability to suppress corona discharge and to prolong the life of paper dielectrics under conditions of elevated temperature and high electrical stress. The improved performance of capacitors made with a methyl cellulose treated paper dielectric will be discussed below.

The water soluble, film-forming material can be applied to the paper by any of the commonly used techniques of coating or surface-treating paper, such as size press coating, blade coating, roll coating, air knife coating, etc. It may be applied either on or off the papermachine, although we prefer the former for reasons of economy.

The coating material may be applied to only one side of the paper, or to both sides. Generally, treatment of both sides is preferred in order to maximize the effect of the treatment and to avoid curl problems. However, in specific instances, as in the preparation of capacitor paper for vacuum metallizing, it is advantageous to apply a heavy treatment to one side of the paper only in order to maximize smoothing of the surface to be metallized. leaving the other side toothy to permit easy penetration of liquid impregnants into the capacitor winding.

The amount of surface treatment which is required to provide significant improvement of electric strength is at least about 2 percent, on a dry weight basis, of the total weight of the treated paper. The maximum amount which can be applied is limited only by the mechanics of application and drying, but we do not find it advantageous or practical to apply amounts greater than about 15 percent of the total weight of the treated paper, and the preferred range is from about 3 percent to about 10 percent.

The following examples are illustrative of the way in which the invention may be practiced and its objectives achieved:

EXAMPLE 1 Methyl cellulose having a degree of substitution of 1.64 to 1.92 was washed by the process previously described until its sodium content was reduced to 12 parts per million. It was then dissolved in water to form a 4% by weight solution by cooling to 5C. This solution had a viscosity of about cps at 20C. The solution was applied to a web fo capacitor tissue by means of a conventional size press located on the papermachine. The basis weight of the resulting treated paper product was adjusted to obtain the same weight obtained prior to the application of the methyl cellulose solution. Both the treated and untreated papers were supercalendered to a thickness of 0.5 mil and a density of 1.0 gm/cc.

It was found that the regular capacitor tissue which had a basis weight of 5.3 lbs/2000 ft and a thickness of 0.51 mils had a dielectric strength of 1540 volts/mil when tested according to ASTM D202-70. The treated paper, having a dry basis weight of 5.3 lbs/2000 ft of which 3.8 percent was methyl cellulose, and a thickness of 0.50 mils, had a dielectric strength of 1,850 volts/mil.

EXAMPLE ll Treated paper was produced as described in Example 1 except that a 3.8 percent by weight solution of methyl cellulose was used to coat a paper which, after supercalendering, had a thickness of 0.4 mils. The paper produced prior to coating had a basis weight of 4.2 lbs/2,000 ft a thickness of 0.4l mils and a dielectric strength of 1,770 volts/mil. The coated paper, also having a basis weight of 4.2 lbs/2,000 ft of which 3.7 percent was methyl cellulose and a thickness of 0.40 mils, had a dielectric strength of 2,080 volts/mil.

The coated and uncoated papers were also used to construct conventional askarel-impregnated 5.0 [.LF capacitors which were stressed to failure with DC voltage. Each capacitor was made by winding metal foil electrodes interleaved with paper, placing the wound body in an enclosure, and evacuating the enclosure and filling it with askarel. The capacitors made with the regular, untreated paper showed an average breakdown voltage of 1,690 volts DC while the units made from the treated paper showed an average breakdown voltage of 2,140 volts DC.

EXAMPLE Ill Treated paper was produced as described in Example 1 except that a hydroxyethylated starch was prepared by washing to a sodium content of parts per million by the method previously described and then dissolved in water by heating to 90C for 30 minutes to produce an 8 percent by weight solution.

The temperature of the solution was adjusted to 50C before application to the paper, at which temperature the viscosity was 65 centipoise. The paper produced prior to application of the starch solution was found to have a basis weight of 4.0 lbs/2,000 ft and a thickness of 0.4 mils and a dielectric strength of 1,520 volts/mil. The treated paper had a dry basis weight of 4.1 lbs/2,000 ft*, of which 7.8 percent was starch, a thickness of 0.4 mils, and a dielectric strength of 2,030 volts/mil.

Conventional 5.0 ,u.F capacitors impregnated with askarel as described above, were produced from both treated and untreated papers and electrically stressed to failure. The units made using untreated paper had an average breakdown value of 1,600 volts DC while the treated units showed an average breakdown value of 1,910 volts DC.

EXAMPLE 1V Treated paper was produced as described in Example I using hydroxyethylated starch as described in Example Ill. The paper produced prior to the application of this solution had a basis weight of 5.0 lbs/2,000 ft, a thickness of 0.5 mils, and a dielectric strength of 1,365 volts/mil. The treated paper had a dry basis weight of 5.1 lbs/2,000 ft, of which 7.7 percent was starch, a thickness of 0.5 mils, and a dielectric strength of l 855 volts/mil.

EXAMPLE V Treated paper was produced as described in Example 1 except that a 7 percent by weight solution of alpha protein was used. This 7 percent solution was prepared as described previously by washing to a sodium content of 14 parts per million and then using ammonium hydroxide to bring the solution to a pH of 8 9 and heating to 60C. The paper made prior to the application of this solution was found to have a basis weight of 4.1 lbs/2,000 ft and a thickness of 0.42 mils, and had a dielectric strength of 1,620 volts/mil. The treated paper had a dry basis weight of 4.1 lbs/2,000 ft of which 6.5 percent was protein, a thickness of 0.42 mils, and a dielectric strength of 1,960 volts/mil.

Conventional 5.0 ,uF askkarel impregnated capacitors as described above, were made from these papers. The units made with the untreated paper had an average breakdown voltage of 2,340 volts DC. The units made with the treated papers had an average breakdown value of 2,650 volts DC.

EXAMPLE V1 Regular capacitor tissue was treated using an offmachine roll coater and a solution of fully hydrolyzed polyvinyl alcohol. The solutuon was previously prepared by washing to a sodium content of 8 parts per million and heating to C. The resulting solution had a viscosity of 35 cps (20C), and a concentration of 3.9 percent by weight solids. The untreated paper had a dry basis weight of 4.3 lbs/2,000 for a thickness of 0.5 mils, and a dielectric strength of 1,780 volts/mil. The treated paper carried 1.8 percent polyvinyl alcohol, had a thickness of 0.5 mils, and had a dielectric strength of 1,950 volts/mil.

1n all the examples set forth above, the basis weight and thickness of the coated paper wsa substantially the same as the basis weight and thickness of the uncoated sample. Nevertheless, the dielectric strength of the coated samples was, in each case, higher than the dielectric strength of the uncoated sheets. From this it follows that if coated and uncoated sheets having the same dielectric strength were produced, the coated sheets would be thinner than the uncoated sheets.

From the above examples it can also be concluded that the percentage increase in dielectric strength of the coated paper, due to the presence of the coating, exceeds the percentage increase in its weight, due to the presence of the coating. The following examples indicate this directly:

EXAMPLE VII Treated paper was produced using a 15 percent solution of acetylated starch prepared as described in Example lll. This solution was heated to 50 C and then applied at the size press of the paper machine to capacitor paper being produced having a basis weight of 4.15 lbs/2,000 ft The basis weight of the resulting treated productwas 4.61 lbs/2,000 ft'-. After producing a quantity of treated paper, the starch solution was discontinued and the amount of cellulose fiber in the sheet was increased, resulting in a basis weight of 4.60 lbs/2,000 ft? The three resulting capacitor papers were supercalendered, and tested as described in Example 1 except that two thicknesses of each paper were stressed to failure using increasing AC voltage. The original paper, having a basis weight of 4.15 lbs/2,000 ft and a thickness of 0.4 mils, had an average breakdown voltage (two sheets) of 1,285 volts AC. The treated paper had a basis weight of 4.61 lbs/2,000 ft of which 13 percent was starch, a thickness of 0.45 mils, and an average breakdown of 1,850 volts AC. This represents, as compared to the original paper, a 44 percent increase in breakdown strength for a 13 percent increase in basis weight. The final paper had a basis weight of 4.65 lbs/2,000 ft all of which was cellulose fiber, a thickness of 0.45 mils, and an average breakdown strength of 1,450 volts AC. This represents as compared to the original paper, a 13 percent increase in breakdown voltage for a 12 percent increase in basis weight.

EXAMPLE VIII Treated paper was produced using a 4 percent solution of methyl cellulose as described in Example I. The solution of methyl cellulose was applied to capacitor paper being made having a basis weight of 3.65 lbs/2,000 ft". The basis weight of the treated paper was 3.8 lbs/2,000 ft After producing a quantity of treated paper, the treatment was discontinued and the amount of cellulose fiber was increased to bring the basis weight to 3.8 lbs/2,000 ft The three papers produced were supercalendered, and the electrical breakdown strength of two thicknesses of each paper was determined with AC voltage. The original paper having a basis weight of 3.65 lbs. and a thickness of 0.35 mils, had an average breakdown voltage (two sheets) of 1,030 volts AC. The treated paper had a basis weight of 3.8 lbs, a thickness of 0.36 mils, and an average breakdown of 1,270 volts. This represents as compared to the original paper, a 23 percent increase in breakdown strength for a 4 percent increase in basis weight. The third paper had a basis weight of 3.8 lbs., all of which was cellulose fiber, a thickness of 0.36 mils, and an average breakdown strength of 1,080 volts. This represents, as compared to the original paper, a percent increase in breakdown voltage for a 4 percent increase in basis weight.

The accompanying drawings illustrate some of the benefits of the present invention.

In the drawings:

FIG. 1 shows the electrical breakdown strengths of untreated insulating paper and such paper treated according to the invention;

FIG. 2 shows the life relationship between capacitors made with both treated and untreated papers;

FIG. 3 shows the dissipation factors of treated and untreated papers; and

FIG. 4 shows the corona resistance of capacitors made with untreated paper and those made with paper coated with methyl cellulose.

FIG. 1 indicates the electrical breakdown strengths of pairs of sheets of capacitor tissues with thicknesses ranging from 03 mils to 0.5 mils. Curve I plots electrical strength against thickness for conventional paper, and Curve II shows the same relationship for our paper coated with a 4 percent by weight solution of methyl cellulose. The breakdown tests were run with 60 cycle AC. voltage, using the apparatus specified in A.S.T.M. Dl4964.

FIG 2 illustrates graphically the ability of the treated papers described herein to resist the degrading effects of high electrical stress. Conventional 0.5 ;LF capacitors were constructed each using two thicknesses of capacitor paper as the dielectric. Three different capacitor papers were employed. Two of the papers selected were regular capacitor tissue of excellent commercial quality, while the third paper was treated by the process described in the examples using a 4 percent by weight methyl cellulose solution. The capacitors were simultaneously impregnated with unmodified askarel in a common vessel using well established practices. After impregnation the units were wired to receive electrical stress and placed in a protective enclosure for life testing. All units were stressed simultaneously at 800 volts AC and allowed to remain so until all units had failed. The times to failure were noted for all units and are shown in FIG. 2.

FIG. 3 illustrates the effect of the treatments described above in the examples on the dissipation factor of dry papers. Superimposed curves numbered I, II, and III represent typical values found with capacitor papers treated with 4 percent methyl cellulose solution, 8 percent starch solution, and untreated capacitor papers. Superimposed curves IV and V represent values experienced with capacitor papers treated with solutions of 3.8 percent polyvinyl alcohol and 7 percent alpha protein. Note that surface coating according to the invention with methyl cellulose and starch have no effect on dissipation factor.

FIG. 4 illustrates the unique ability of papers treated with methyl cellulose to suppress corona discharges in capacitors when subjected to high voltages.

Referring to FIG. 4, curves I, II and III illustrate the effect of electrical stress on the dissipation factor of regular capacitor papers, absorbent additive capacitor papers, and methyl cellulose treated papers, respectively.

The capacitors used were conventional l.O ,u.F askarel-impregnated units using two sheets of 0.4 mil papers as the dielectric material. Curve I (regular capacitor paper) shows the typical stress dependence of the dissipation factor of capacitors. The initially high dissipation factor values are due to movements of ionic impu rities found in small amounts in the impregnating liquid phase of the capacitors when AC stress is applied. As stress is increased, the rate of motion of the ionic movements is increased and the resulting dissipation factor is lower. Curve I in FIG. 4 shows the lowest value of dissipation factor for capacitors made with conventional capacitor papers to occur in the range of 300 to 400 volts/mil. As the AC stress is increased above 400 volts per mil on regular paper, we observe an increase of the dissipation factor which indicates the dielectric cannot adequately withstand the applied stress and that corona discharge on a small scale is occurring, resulting in the generation of more ionic impurities from the electrical discharges. As this process continues, the dissipation factor rises at an accelerating rate.

Again referring to FIG. 4 we note Curve II representing the newer, commercially used additive capacitor papers manufactured in accordance with US. Pat. Nos. 3.090,705; 3,480,847; and 3,555,377. These papers are conventional capacitor papers in which have been incorporated small percentages of finely divided mineral adsorbents. These materials have the desirable effect of scavenging and holding by adsorptive forces the previously mentioned ionic impurities from the liquid dielectric, thus removing them from the liquid phase of a capacitor and immobilizing them when low A.C. stress is applied. This accounts for the flat shape of Curve II. However, this beneficial scavenging effect does not prevent electrical discharges from occurring as stresses above 400 volts/mil are encountered. Thus, while the dissipation factor of capacitors made with additive papers is not stress dependent in the low stress range, and even though a cleaning of the liquid phase of a capacitor is observed, there is little evidence that the additive papers can be expected to resist corona discharges.

Curve III illustrates the effect of electrical stress on the dissipation factor of capacitors made using our methyl cellulose treated paper. Note that from a slightly higher initial dissipation factor value, the curve drops rapidly to its lowest dissipation factor value at about 300 volts/mil. Increasing stress up to 500 volts/- mil does not cause an increase in dissipation factor and then only slightly up to extreme stresses of 800 volts/- mi] and above.

The invention has been shown and described in preferred form only, and by way of example, and many variations may be made in the invention which will still be comprised within its spirit. It is understood, therefore, that the invention is not limited to any specific form or embodiment except insofar as such limitations are included in the appended claims.

What is claimed is:

1. Electrical insulating paper bearing a surface coating of a material insoluble in liquid dielectrics, such as chlorinated biphenyls and mineral oil, and being substantially free of alkali metals, said coating material being insoluble in water within one temperature range but soluble in water within a different temperature range, and said coating material being present to the extent of between 2 and percent of the total weight of the coated paper, said coated paper being thinner than otherwise identical uncoated paper having the same dielectric strength, and said coated paper having a percentage increase in dielectric strength greater than the percentage increase in its weight due to the presence of the coating.

2. Electrical insulating paper as defined in claim I wherein only one surface of the paper is coated.

3. Electrical insulating paper as defined in claim 1 wherein both surfaces of the paper are coated.

4. Electrical insulating paper as defined in claim I wherein said coating material contains no more than 200 parts per million of alkali metals.

5. Electrical insulating paper as defined in claim 1 wherein said coating material contains no more than parts per million of alkali metals.

6. Electrical insulating paper as defined in claim 1 wherein said coatang is present to the extent of between 3 and 10 percent of the total weight of the coated paper.

7. Electrical insulating paper as defined in claim 1 wherein said coating material is methyl cellulose.

8. Electrical insulating paper as defined in claim 7 wherein a 2 percent aqueous solution of said methyl cellulose has a viscosity at 20C of about 10 to centipose, and a degree of substitution between 1.64 and 1.92.

9. Electrical insulating paper as defined in claim 1 wherein said coating material is starch.

10. Electrical insulating paper as defined in claim 9 wherein a 10 percent aqueous solution of said starch has a viscosity at 20C. below about 200 centipoises.

11. Electrical insulating paper as defined in claim 1 wherein said coating material is polyvinyl alcohol.

12. Electrical insulating paper as defined in claim 11 wherein said polyvinyl alcohol is fully hydrolyzed, and a 4 percent aqueous solution of said polyvinyl alcohol has a viscosity at 20C between about 25 and centipoise.

13. Electrical insulating paper as defined in claim 1 wherein said coating material is protein.

14. Electrical insulating paper as defined in claim 11 wherein said protein is alpha protein.

15. Electrical insulating paper as defined in claim 1 wherein said coating material is selected from the group consisting of methyl cellulose, starch, polyvinyl alcohol, and protein.

16. A method of making electrical insulating paper, comprising the steps of a. providing a material insoluble in liquid dielectrics, such as chlorinated biphenyls and mineral oil, said material being soluble in water within a first temperature range but insoluble in water within a second temperature range,

b. washing the material in water within said second temperature range to remove substantially all the alkali metals from the material,

c. dissolving the washed material in water within said first temperature range to produce an aqueous solution,

d. coating the surface of an electrical insulating paper web with the aqueous solution so that the coating material is present to the extent of between 2 and 15 percent of the total dry weight of the coated paper and e. drying the paper.

17. A method as defined in claim 16 including adding cations to the wash water of step (b).

18. A method as defined in claim 17 wherein the cations are added by adding magnesium sulfate or sulfuric acid in an amount of about 0.01 to 0.l percent by weight of the wash water.

19. A method as defined in claim 16 wherein the material is finely divided prior to washing.

20. A method as defined in claim 16 wherein the washing step (b) is carried on until the alkali metal content of the material has been reduced to no more than 200 parts per million.

21. A method as defined in claim 16 wherein the washing step (b) is carried on until the alkali metal content of the material has been reduced to no more than 20 parts per million. 

1. ELECTRICAL INSULATING PAPER BEARING A SURFACE COATING OF A MATERIAL INSOLUBLE IN LIQUID DIELECTRICS, SUCH AS CHLORINATED BIPHENYLS AND MINERAL OIL, BEING SUBSTANTIALLY FREE OF ALKALI METALS, SAID COATING MATERIAL BEING INSOLUBLE IN WATER WITHIN ONE TEMPERATURE RANGE BUT SOLUBLE IN WATER WITHIN A DIFFERENT TEMPERATURE RANGE, AND SAID COATING MATERIAL BEING PRESENT TO THE EXTENT OF BETWEEN 2 AND 15 PERCENT OF THE TOTAL WEIGHT OF THE COATED PAPER, SAID COATED PAPER BEING THINNER THAN OTHERWISE IDENTICAL UNCOATED PAPER HAVING THE SAME DIELECTRIC STRENGTH, AND SAID COATED PAPER HAVING A PERCENTAGE INCREASE IN DIELECTRIC STRENGTH GREATER THAN THE PERCENTAGE INCREASE ON ITS WEIGHT DUE TO THE PRESENCE OF THE COATING.
 2. Electrical insulating paper as defined in claim 1 wherein only one surface of the paper is coated.
 3. Electrical insulating paper as defined in claim 1 wherein both surfaces of the paper are coated.
 4. Electrical insulating paper as defined in claim 1 wherein said coating material contains no more than 200 parts per million of alkali metals.
 5. Electrical insulating paper as defined in claim 1 wherein said coating material contains no more than 20 parts per million of alkali metals.
 6. Electrical insulating paper as defined in claim 1 wherein said coatang is present to the extent of between 3 and 10 percent of the total weight of the coated paper.
 7. Electrical insulating paper as defined in claim 1 wherein said coating material is methyl cellulose.
 8. Electrical insulating paper as defined in claim 7 wherein a 2 percent aqueous solution of said methyl cellulose has a viscosity at 20*C of about 10 to 25 centipose, and a degree of substitution between 1.64 and 1.92.
 9. Electrical insulating paper as defined in claim 1 wherein said coating material is starch.
 10. Electrical insulating paper as defined in claim 9 wherein a 10 percent aqueous solution of said starch has a viscosity at 20*C. below about 200 centipoises.
 11. Electrical insulating paper as defined in claim 1 wherein said coating material is polyvinyl alcohol.
 12. Electrical insulating paper as defined in claim 11 wherein said polyvinyl alcohol is fully hydrolyzed, and a 4 percent aqueous solution of said polyvinyl alcohol has a viscosity at 20*C between about 25 and 125 centipoise.
 13. Electrical insulating paper as defined in claim 1 wherein said coating material is protein.
 14. Electrical insulating paper as defined in claim 11 wherein said protein is alpha protein.
 15. Electrical insulating paper as defined in claim 1 wherein said coating material is selected from the group consisting of methyl cellulose, starch, polyvinyl alcohol, and protein.
 16. A method of making electrical insulating paper, comprising the steps of a. providing a material insoluble in liquid dielectrics, such as chlorinated biphenyls and mineral oil, said material being soluble in water within a first temperature range but insoluble in water within a second temperature range, b. washing the material in water within said second temperature range to remove substantially all the alkali metals from the material, c. dissolving the washed material in water within said first temperature range to produce an aqueous solution, d. coating the surface of an electrical insulating paper web with the aqueous solution so that the coating material is present to the extent of between 2 and 15 percent of the total dry weight of the coated paper and e. drying the paper.
 17. A method as defined in claim 16 including adding cations to the wash water of step (b).
 18. A method as defined in claim 17 wherein the cations are added by adding magnesium sulfate or sulfuric acid in an amount of about 0.01 to 0.1 percent by weight of the wash water.
 19. A method as defined in claim 16 wherein the material is finely divided prior to washing.
 20. A method as defined in claim 16 wherein the washing step (b) is carried on until the alkali metal content of the material has been reduced to no more than 200 parts per million.
 21. A method as defined in claim 16 wherein the washing step (b) is carried on until the alkali metal content of the material has been reduced to no more than 20 parts per million. 